1. Field of the Invention
The present invention relates to thyristors (including a light-activated thyristor) with an overvoltage self-protection function of punchthrough type and, more particularly, to a thyristor manufacturing method which can precisely realize the breakover voltage at which the punchthrough occurs and is particularly used for manufacturing thyristors used for a high-voltage conversion application in direct-current power transmission.
2. Description of the Related Art
In the field of high-power thyristors used for high-voltage converting applications, thyristors with overvoltage self-protection function have been developed. Many of the thyristors with overvoltage protection are of the avalanche type. This type of thyristor is described in "Controlled Turn-on Thyristors", by VICTOR A. K. TEMPLE, IEEE, Trans. Electron Devices, ED-30, pp. 816-824 (1983) GE.
In the case of this avalanche type, formed in a part of the P gate-base layer of a thyristor of PNPN structure is a region in which avalanche breakdown is more likely to occur than other parts. In the operation of overvoltage protection, the avalanche-breakdown first occurs in the region by means of a transient voltage at the rise of an overvoltage and thus a nondestructive current flows through the region with the result that a pilot thyristor is turned on and subsequently a main thyristor is turned on to decay the overvoltage, thereby protecting the thyristor.
In an intermediate process of manufacture of avalanche type devices a gate-base region for avalanche breakdown is formed. When completed, therefore, the devices will have varying breakover voltages because of variations in material or in process. Since the avalanche voltage has positive temperature dependency, the breakover voltage naturally has temperature dependency. This means that the breakover voltage is higher at room temperature than at high temperature, making it difficult to design thyristor devices, particularly in the respects of their withstanding voltages and di/dt ratings.
A thyristor with overvoltage self-protection of punchthrough type is disclosed in a paper entitled "LASER TRIMMING OF THYRISTORS TO ADD AN OVERVOLTAGE SELF-PROTECTED TURN-ON FEATURE" by J. X. Przybysz, IEEE 1985, pp. 463-468.
FIG. 3 shows a modification of a punchthrough type thyristor illustrated in FIG. 2 on page 464 of the above paper, to which the present invention is not applied. This device comprises a main thyristor having a four-layer structure comprised of a P emitter layer 1, an N base layer 2, a P gate-base layer 3 and N emitter layers 4a; a pilot thyristor having a four-layer structure comprised of P emitter layer 1, N base layer 2 and P gate-base layer 3 which are all common to the main thyristor, and N emitter layers 4b surrounded with N emitter layers 4a; and a recess 20 of a gate portion surrounded with the pilot thyristor. Reference numeral 6 denotes an anode electrode, 7 a cathode electrode, 8 a gate electrode, and 9 an amplifying gate electrode (a cathode electrode of the pilot thyristor).
In this punchthrough type, a space-charge layer 5 (the region between broken lines 5a, 5b) is formed in a junction 11 between P gate-base layer 3 and N base layer 2 by a forward voltage applied to the thyristor, and this region 5 extends with an increase in the forward voltage. When the forward voltage is increased to the breakover voltage (hereinafter referred to as the self-protection breakover voltage) at which the self-protective operation starts, the boundary 5a of space-charge layer 5 reaches bottom 20a of the recess of P gate-base layer 3 so that the punchthrough occurs. Consequently, the current flowing through junction 11 acts as a gate current of the pilot thyristor (1, 2, 3, 4b) to turn it on. Immediately after the pilot thyristor is turned on, the main thyristor (1, 2, 3, 4a) is safely turned on, thereby protecting the main thyristor.
Space-charge layer extending to P gate-base region 3 becomes narrow because the impurity concentration of layer 3 is much higher than that of N base 2. Although being possible in the stage of laboratory, therefore, the precise control of the self-protection breakover voltage is very difficult to attain from the viewpoint of mass productivity.
FIG. 4A shows a structure model of a punchthrough type thyristor, while FIG. 4B is a graph showing the relationship between a distance W.sub.PB from bottom 20a of the recess to junction 11 and a breakover voltage Vbo at which the punchthrough occurs in P gate-source layer 3. As can been seen from FIG. 4B, the breakover voltage Vbo varies greatly as the distance W.sub.PB varies slightly. It will thus be found that the precise control of the self-protection breakover voltage is difficult to attain. (It is to be noted that, in FIG. 4B, W.sub.PB is represented by the linear scale, while Vbo is represented by the logarithmic scale.) The distance W.sub.PB varies with the profile of the impurity concentration of the P gate-base layer.
To solve the above problems, it is required to determine the depth of the recess for each of thyristor pellets, for example, by monitoring them with a voltmeter, in order for the P gate-base layer below the recess to accurately cause the punchthrough at a self-protection breakover voltage.
As described above, the problems with the conventional method of manufacturing thyristors with overvoltage self-protection are that: it is difficult to determine the self-protection breakover voltage with precision in intermediate processes of manufacture; and thus the complete devices have great variations in their breakover voltage. The method utilizing the voltmeter for monitor can solve the above problems, but is not suitable for mass production because many manufacturing processes are involved.